1. Field of the Invention
This invention relates to banded radial run-flat tires and, more particularly, to self-energized means for enhancing the load-carrying capabilities of such tires in the deflated or partially deflated condition.
2. Background of the Invention
An area of tire technology which has engaged investigators over the years is the run-flat pneumatic tire concept. A run-flat tire is one designed to support a vehicle for operation even if the tire has partially or totally lost its inflation pressure. The advantages of such a tire in safety, convenience, cost, and weight and space saving are obvious. A recent successful development in the art of run-flat tires is the band-reinforced radial tire invented by the inventor in the present application, which banded tire is the subject of U.S. Pat. No. 4,111,249, assigned to the assignee of the present invention. The run-flat tire with which the load enhancement system of the subject invention is used is the tire disclosed in that Markow patent, U.S. Pat. No. 4,111,249, which is hereby incorporated herein by reference.
The banded run-flat tire is a pneumatic radial tire having a casing with a crown and sidewalls extending from the crown on either side to annular beads which, in a conventional way, are used to mount the tire in a sealed relationship on the rim of a wheel. In the design, the band element, which preferably is a thin structural ring of high-strength steel or a fiber/epoxy composite, is incorporated circumferentially into the crown of the tire under the tread thereof. Radial tires, as is well known, have a multiplicity of closely spaced radial reinforcing cords or wires in the sidewalls of the tire casing. In the tire disclosed in the above-referenced patent to Markow, the radial cords or wires function as spoke-like reinforcing elements to stabilize the circumferential band. When the tire is deflated, the radial spoke-like elements and the band stabilized thereby form a load-sustaining structure analogous to an elastic arch. In the design, the band receives vertical, drag, and side loads from the road or ground surface, and carries those loads in compression and bending; the radial spoke-like elements act as tension members to support the axle. A prime function, also, of the closely spaced radial elements is to stabilize the thin band against buckling.
A critical factor in the bended radial tire concept is the structural interaction of the band with the radial elements in the tire sidewalls. In the pressurized condition of operation with the sidewalls (and the radial elements therein) bulging outwardly in the normal curved configuration, the band contributes between 5-10% to the tire's spring rate. Unpressurized, the sidewalls tend to straighten out in all but the ground contact area and the radial elements become vertical tensile elements creating an arch structure in which 80-95% of the load is carried in the band. Overload imposes stresses on the band that makes it vulnerable to fatigue.
I have found that a small outward force on the sidewalls to cause them to bulge and thereby reduce the length of sidewall extension results in a reduction in band deflection that provides a significant increase in load-carrying ability up to 50% for a banded tire in the deflated condition.
In my invention, the small outward force on the sidewalls to cause them to bulge is provided by an undersized inner tube stretched to lie within the drop center of the rim. This inner tube is inflated to a pressure that is significantly lower than the normal inflation pressure in the tire and, although the pressure in the tube is not adequate to support the vehicle when normal tire inflation pressure is lost, the tube pressure is sufficient to provide the necessary outward force on the sidewalls that impart the required curvature to the spoke-like radial elements therein. Typically, in this invention, if the tire is inflated to a pressure of about 30 psi, the tube will be inflated to a pressure of about 3-5 psi. Because of this pressure differential, in operation with the tire at normal pressure, the tube is compressed to less than a quarter of its expanded volume such that it rides in the drop center of the wheel rim out of contact with the tire crown and substantially all of both sidewalls. Thus, there is virtually no interaction of the inner tube with the tire in operation until normal tire inflation pressure is lost. Because of the band, the banded tire is substantially invulnerable to punctures through the tread. Thus, a puncture of the inner tube due to a retained object will not be experienced. Puncture of the inner tube by a retained object is a recognized weakness in prior inner tube systems.